SENSORIMOTOR STATE OF THE CONTRALATERAL LEG AFFECTS IPSILATERAL MUSCLE COORDINATION OF PEDALING

The objective of this study was to determine if independent central pa ttern generating elements controlling the legs in bipedal and unipedal locomotion is a viable theory for locomotor propulsion in humans. Coo rdinative coupling of the limbs could then be accomplished through mec hanical interactions and ipsilateral feedback control rather than thro ugh central interlimb neural pathways. Pedaling was chosen as the loco motor task to study because interlimb mechanics can be significantly a ltered, as pedaling can be executed with the use of either one leg or two legs (cf. walking) and because the load on the limb can be well-co ntrolled. Subjects pedaled a modified bicycle ergometer in a two-legge d (bilateral) and a one-legged (unilateral) pedaling condition. The lo ading on the leg during unilateral pedaling was designed to be identic al to the loading experienced by the leg during bilateral pedaling. Th is loading was achieved by having a trained human ''motor'' pedal alon g with the subject and exert on the opposite crank the torque that the subject's contralateral leg generated in bilateral pedaling. The huma n ''motor'' was successful at reproducing each subject's one-leg crank torque. The shape of the motor's torque trajectory was similar to tha t of subjects, and the amount of work done during extension and flexio n was not significantly different. Thus the same muscle coordination p attern would allow subjects to pedal successfully in both the bilatera l and unilateral conditions, and the afferent signals from the pedalin g leg could be the same for both conditions. Although the overall work done by each leg did not change, an 86% decrease in retarding (negati ve) crank torque during limb flexion was measured in all 11 subjects d uring the unilateral condition. This corresponded to an increase in in tegrated electromyography of tibialis anterior (70%), rectus femoris ( 43%), and biceps femoris (59%) during flexion. Even given visual torqu e feedback in the unilateral condition, subjects still showed a 33% de crease in negative torque during flexion. These results are consistent with the existence of an inhibitory pathway from elements controlling extension onto contralateral flexion elements, with the pathway opera ting during two-legged pedaling but not during one-legged pedaling, in which case flexor activity increases. However, this centrally mediate d coupling can be overcome with practice, as the human ''motor'' was a ble to effectively match the bilateral crank torque after a longer pra ctice regimen. We conclude that the sensorimotor control of a unipedal task is affected by interlimb neural pathways. Thus a task performed unilaterally is not performed with the same muscle coordination utiliz ed in a bipedal condition, even if such coordination would be equally effective in the execution of the unilateral task.